Yeah, you're probably right. My experiences with constant speed props is pretty limited. Flew em about 8 years ago, but only for about 15-20 hours. Most of my experience is with jets. For this question, though, it should be immaterial, however, because if constant speed props don't lose the ability to produce thrust as velocity increases, they would still suffer a reduction in Vy with altitude due to the shift in the power required curve. In other words, the "JET" explanation above would be the same phenomenon in a constant speed prop, but a fixed prop would have 2 factors affecting it.
Oh, well. That'll teach me. Think before you speak (or type!)
